A man who says he saw Trayvon Martin shot dead claims that the Florida teenager and his killer, George Zimmerman, were scuffling on the ground at the time with one on top of the other.

The first eyewitness account of the 17-year-old's final moments emerged on Thursday night more than a month after the boy lost his life in an altercation with a neighbourhood watch leader in a gated community in Sanford.

The anonymous man said he reported to police details of what he saw on the evening of 26 February, which included watching the gunman walking away from the fight apparently uninjured.

It contradicts an allegation from Zimmerman's father earlier in the day that the unarmed black teenager broke his son's nose during the incident and also left him with bloody injuries from slamming the man's head repeatedly on to a concrete pavement. The eyewitness says he saw no blood and that the entire confrontation took place only on grass.

"I saw two men on the ground, one on top of the other. I felt they were scuffling and I heard gunshots which to me were more like pops," he said in an interview broadcast on CNN's Anderson Cooper 360, his voice disguised to protect his identity.

"I don't know if was an echo but it definitely made more than one pop."After the larger man got off there was a boy, obviously now dead, on the ground facing down.

"It was dark. I can't say I watched him get up, but in a couple of seconds or so he was walking towards where I was watching and I could see him a little bit clearer. It was a Hispanic man. He didn't appear hurt or anything else. He just kind of seemed very worried with his hand up to his forehead."

The man said that before opening his window and looking out, he had heard angry voices outside. "There was a loud, predominant voice. I couldn't hear the words but this is not a regular conversation," he said. "This is someone aggressively yelling at someone."

He said there was a lull but the argument resumed and that was when he decided to see what was going on.

"I'm thinking something horrible is happening. I heard the yell for help and another excruciating kind of a yell, it didn't even sound like a yell, it sounded so painful," he said.

The Sanford police department, which has been criticised for not arresting Zimmerman, 28, would not confirm the witness's account, referring questions to the office of the Florida state attorney Angela Corey, who was appointed last week to oversee the case.

Nobody was available at Corey's office to comment.

But Daryl Parks, attorney for Trayvon's parents Sybrina Fulton and Tracy Martin, said the story "answers a lot of questions".

He said: "It is very clear this witness saw what happened and clearly indicated the person that he saw that did the shooting. The other part that really strikes out for me is that he seems to have not seen the apparent injuries from this altercation that Mr Zimmerman claims that he suffered.

"It seems clear that Zimmerman's statement about Trayvon following him back to the truck is totally untrue because from this witness's statement all the interaction happened in one particular area, so I think Mr Zimmerman will be arrested very soon."

CCTV video which shows Zimmerman hours after the teenager's death has also cast doubt on his claims that he was injured in a vicious fight with the victim. Earlier reports had suggested Trayvon attacked Zimmerman first, with the bloodied gunman pulling the trigger in self-defence.

But in the footage, first aired by ABC news, Zimmerman's head and face are clearly visible and there appears to be no sign of any wounds. There are no obvious indications of blood on the front of his T-shirt that could indicate evidence of a broken nose.

• This article was amended on 30 March 2012. The original photo caption was expanded to make clear that the photo of Trayvon Martin, aged 17 at the time of his death, was a historic image. A more recent photo is not currently available from picture agencies.